Giani worked in many Italian cities throughout his long career. He was trained by Carlo Bianchi and Antonio Galli Bibiena in Pavia and in 1778 moved to Bologna to study with Domenico Pedrini and Ubaldo Gandolfi. He spent the years between 1780 and 1786 in Rome studying with Pompeo Batoni, Giuseppe Antolini, and Christoph Unterberger at the Accademia di San Luca. During this time he won a prize for painting at the Accademia and a second prize from the Accademia di Belle Arti di Parma for his Samson and Delilah.
Giani established himself as a an important decorative painter; he contributed to projects in Forli, Venice, Ravenna, Ariccia, Bologna, Faenza, and Rome. He had an international clientele and worked for Catherine II of Russia in 1788 and in Paris for Napoleon at the Tuilleries and Malmaison in 1803. In 1805 he settled more or less permanently in Rome and contributed to the decoration of the Palazzo di Spagna in 1806 and the Palazzo Quirinale in 1812-1813. He was elected a member of the Accademia di San Luca in 1811 and to the Congregazione dei Virtuosi of the Pantheon in 1819. His last large decorative project was the Teatro della Valle in Rome in 1821. (Lisa Dickinson Michaux, Visions of Antiquity: Neoclassical Figure Drawings. Los Angeles County Museum of Art and The Minneapolis Institute of Arts, 1993: p. 280)